Typical adhesives used for multilayer pipes containing an aluminum layer show high peel strength initially after pipe production, but significantly decreased peel strength after 2500 hours of temperature cycling test. There is a need for compositions that can be used as adhesives for multi-layered pipe manufacturing, and which provide improved long term peel strength and peel strength retention between pipe layers, and especially between a layer formed from an ethylene-based polymer and a layer formed from a metal, such as aluminum.
International Publication No. WO 01/55275 discloses laminated films, comprising at least one structural layer, at least one adhesive layer, and optionally at least one barrier layer, made by a high speed or fast quench film process. The adhesive layer is formed from a composition comprising the following: a) a polyethylene selected from the group consisting of conventional-HDPE, conventional-MDPE, conventional-LLDPE, conventional-VLDPE, LDPE, and a blend thereof; b) from 5 to 35 weight percent, based on the total weight of a) plus b) plus c), of an acid-grafted substantially linear polyethylene; and c) optionally, up to 30 weight percent of a hydrocarbon elastomer. The acid grafting agent is an unsaturated carboxylic acid or a derivative thereof, and the level of grafting is such that the total amount of grafting agent in the total composition (a) plus (b) plus (c)) is from 0.01 to 3 weight percent.
Japanese Publication No. 08-208915 (Abstract) discloses an adhesive polyethylene composition, prepared by grafting an unsaturated carboxylic acid, under melt kneading conditions, onto a composition comprising 99-40 weight percent of an ethylene/α-olefin copolymer (A) and 1-60 weight percent of a linear low-density polyethylene (B), and in the presence of a reaction initiator. Component A is obtained by copolymerizing ethylene with an α-olefin in the presence of a single-site catalyst (a combination of a metallocene compound of a group IV or V transition metal with an organoalumium compound and an ionic compound). Component B is prepared by copolymerizing ethylene with an α-olefin in the presence of a Ziegler or chromium catalyst. The composition may be further mixed with an ethylene/propylene copolymer rubber to form a second composition, disclosed as having improved low-temperature properties, and which can be used for coating cables and steel pipes.
U.S. Pat. No. 4,684,576 discloses succinic acid or succinic anhydride grafts of HDPE, which are blended with ungrafted LLDPE and with ungrafted LDPE to form blends having useful adhesive properties. The succinic acid or succinic anhydride groups are provided by grafting, respectively, maleic acid or maleic anhydride onto HDPE. This reference also discloses laminates of the blends on various metals and polymers.
European Patent Application No. EP 1316598A1 discloses an adhesive polymer composition comprising the following: a) a non-elastomeric polyethylene, in an amount of 40 to 97 weight percent of the total composition, and b) an elastomer. Component a) is produced in a process using a single-site catalyst, and component a), or components a) and b), are grafted with an acid grafting agent. The reference also discloses a multilayer pipe, comprising an adhesive layer which comprises said adhesive polymer composition. Preferred elastomers include ethylenevinylacetate, ethylenemethylacrylate, ethylenemethylmethacrylate, ethylenepropylacrylate and ethylenebutylacrylate.
European Patent Application No. EP 0896044A1 discloses an adhesive resin composition, which contains at least a partially or wholly graft-modified α-olefin/aromatic vinyl random copolymer, which has a graft quantity of an unsaturated carboxylic acid, or a derivative thereof, ranging from 0.01 to 30 percent by weight. This reference also discloses a laminate comprising: (i) a polyester resin layer or a polycarbonate layer, (ii) an adhesive layer formed from the above-mentioned adhesive resin composition, and (iii) a saponified olefin/vinyl acetate copolymer layer.
U.S. Pat. No. 6,617,019 discloses a composite protective coating for metal substrates, such as steel, used in fabricating metal pipe or conduit. The coating, when applied to a flat metal surface, retains its bond to the metal through cold forming processes, such as those used in making ribbed steel pipe. The coating resembles a sandwich of a reinforcing material layer between two adhesive films. An adhesive layer may be formed from a copolymer of ethylene and acrylic acid, or a hydroxphenoxy ether polymer. Other suitable polymers include homopolymers and interpolymers of ethylene modified with a reactive carboxylic acid anhydride.
European Patent Application No. 0791628A1 discloses an adhesive composition comprising a specified grafted, long-chain branched ethylene-α-olefin copolymer, and either a tackifier, or a specified ethylene-vinyl acetate copolymer, and where the density, MFR, crystallinity and graft ratio, thereof, are within specific ranges. A composition containing a tackifier can be used in a laminate, and is disclosed as having excellent adhesive strength, even in a high-temperature atmosphere. The invention also provides an adhesive composition comprising a grafted long-chain ethylene-α-olefin copolymer and an olefinic elastomer. This composition can be used as a film layer for adhesion to metals or highly polar materials.
There remains a need for adhesives with improved retained adhesion to both aluminum and polyethylene. This need is critical in multi-layer pipes containing both polyolefin layers and metal layers, and in such pipes which are used for hot water applications. There is a further need for pipes with improved adhesion and peel strength after temperature cycling, and improved security and better long-term behavior. There is also need for adhesives that can be made in a compounding extruder process, in a solid phase grafting process, and/or in reactive extrusion process. Some of these needs and others have been met by the following invention.